Solid state batteries, such as thin film batteries, are being rapidly developed for many applications. The energy density and specific energy of a battery, corresponding to the energy capacity of the battery per unit volume and weight, respectively, are notable performance measures. Generally, solid state and thin film batteries can provide higher energy density and specific energy than liquid containing batteries. In small sizes, solid state batteries are often fabricated by microelectronic processing techniques, and may be used in applications such as for example, portable electronics, medical devices, and space systems. In larger sizes, the batteries can be used to power electric cars or store electrical power in a home or electrical grid.
In general, solid state batteries may be formed by creating a reservoir of the charge carrier, such as lithium (Li), on the anode side of the device by depositing metallic Li. In other cases, this reservoir of charge carriers is not created separately, as the device relies on the charge carriers already in the cathode. In such cases, the Li anode layer may be replaced with a metallic layer to provide the anode area to complete the electrochemical devices layout. Some of the materials may include transition metals, such as Cu, and alloy materials, capable of forming an intricate interface with the electrolyte layer below.
Certain problems are observed with conventional solid state batteries formed minus a separately deposited anode/negative electrode layer. For example, the de-intercalated Li from the cathode side (e.g., LiCoO2) may press the structure anisotropically at the electrolyte-anode/Cu interface, leading to stress induced breaking/cracking of the metal anode layer as the volume of Li detaches the Cu from the interface. Such cracking can lead to a direct path of oxidants, leading to loss of limited charge carriers, and thus, the battery capacity. The stress results from the fact the Li from the cathode side does not intercalate or readily alloy with the anode material or the non-Li metallic anode (current collector). Other consequences of this cracking are an increase in overall device impedance from the increased electrical and electrochemical contact resistances (not continuous interface), as well as the impedance from the reacted/oxidized charge carriers (e.g., in the form of LiOH, Li2CO3, etc.), leading to device performance degradation. As such, interface engineering is necessary with conventional devices.